1. Field of the Invention
The present invention relates to a magnetron, and more particularly to a magnetron which is adapted to operate under large electrical energy, for example, electrostatic capacitance between inner circumferential surfaces of holes formed at magnetic pole pieces and outer circumferential surfaces of upper and lower end shields so as to improve oscillation efficiency.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a conventional magnetron. The magnetron comprises an anode cylinder 1, an upper magnetic pole piece 2 positioned above the anode cylinder 1 which has an upper hole 2' and a lower magnetic pole piece 3 positioned under the anode cylinder 1 which has a lower hole 3'. Anode vanes 4 are radially arranged and attached to the inner circumferential surface of the anode cylinder 1. The anode vanes 4 define at inner ends thereof a central space having diameter equal to that of holes 2' and 3' of the magnetic pole pieces 2 and 3. A center bar 8 having upper and lower end shields 6 and 7 is positioned in the hole of the vanes 4. A coil filament 5 is inserted on the center bar 8 between the end shields 6 and 7.
The magnetic pole pieces 2 and 3 is made of ferromagnetic material so as to focus or direct the magnetic flux in a space between the anode vanes 4 and the coil filament 5. End spaces 10 and 11 are provided between the upper and lower magnetic pole pieces 2 and 3 and the anode vanes 4, respectively.
Upon supplying the magnetron with electric power in order to generate an electromagnetic wave, high voltage of 4 KV is applied between the coil filament 5 of the center bar 8 and the anode vanes 4 and thermions (thermal electrons) having magnetic flux density of 1750 gauss are emitted from the coil filament 5. The emitted thermions are in a cycloidal orbit in the space 9 between the filament 5 and the anode vanes 4 by electric field and magnetic field and thus generate microwave energy. The microwave energy is directed to a waveguide (not shown) through an output antenna (not shown).
Oscillation efficiency of the magnetron is generally increased in proportion to increase of energy Q value (Q factor) of the resonator. The oscillation efficiency is affected by various factors such as magnetic flux distribution in the space 9, ratio of radius of hole of the anode vanes to radius of the coil filament, and the size and shape of the holes of the magnetic pole pieces.
Also, the Q value of the magnetron is increased in proportion to increase of electrical property or energy, for example, electrostatic capacitance between outer circumferential surfaces of end shields and inner circumferential surface of magnetic pole pieces and thus oscillation efficiency is also improved in proportion to the increase of electrostatic capacitance.
Referring to FIG. 2, there is shown an enlarged fragmentary section of the magnetic pole pieces and the end shield shown in FIG. 1. In the drawing, if it is assumed that an imaginary reference plane is z, a distance between the imaginary reference plane and an imaginary center plane of outer portion of the end shield 6 is x, and a distance between the imaginary reference plane and an imaginary center plane of the hole 2' of the magnetic pole pieces 2 is y, the distance x does not coincide with the distance y. Therefore, the electrical property or energy between the outer circumferential surfaces of the end shields and the inner circumferential surface of the magnetic pole pieces can not be large.
Accordingly, if the imaginary center plane of the holes 2' and 3' of the magnetic pole pieces 2 and 3 comes close to the imaginary central plane of the outer portion of the end shields 6 and 7 which are disposed on opposite ends of center bar 8, that is, the distance y is shortened to equal the distance x in order to increase electrostatic capacitance between the end shields 6 and 7 and the magnetic pole pieces 2 and 3, since inner surfaces of the magnetic pole pieces 2 and 3 are in contact with or too close to the vanes 4, electric field which is to be directed to the vanes 4 from the coil filament 5 curves toward the magnetic pole pieces 2 and 3 at inner edge of the vane 4, thereby causing unsafe oscillation of the magnetron.
Contrary to the above-mentioned case, if the imaginary central plane of the outer portion of the end shields 6 and 7 comes close to the imaginary center plane of the holes 2' and 3' of the magnetic pole pieces 2 and 3, that is, the distance x is lengthened to equal the distance y, since the coil filament 5 is lengthened so that thermions emitted from both ends of the coil filament 5 are directed not toward vanes 4 but toward end spaces 10 and 11 defined between the vanes 4 and the magnetic pole pieces 2 and 3, the anode cylinder 1 is excessively heated.
In addition, although spacings between inner circumferential surfaces of the holes 2' and 3' of the magnetic pole pieces 2 and 3 and outer circumferential surfaces of the end shields 6 and 7 can be reduced in order to increase electrostatic capacitance, the reduction of spacings causes the insulation between the surfaces of the holes and surfaces of the end shields to be broken or damaged.